Hundreds of millions of internal combustion engines employ exhaust valves, most also employing intake valves, in positive displacement working chambers of those engines to ventilate them. A positive displacement chamber is a substantially sealed variable volume enclosure. Most such chambers are piston-and-cylinder with a cylinder head in which the valves are located. Typically, such a valve is actuated by a cam follower driven by a cam that is driven by the engine crankshaft. The timing of the valve's actuation is according to the cam's rotational position with respect to the follower and the crankshaft.
The cam that is most often used can be called a radial cam, and runs the length of an engine's cylinder head. Thus, it occupies a sizable footprint on the cylinder head. A radial cam is mounted to rotate about an axis and has lobes to shove its followers radially away from the axis, to actuate the engine valves. A radial cam tends to be massive, needing to be both strong and torsionally stiff to fulfill its service life. Such a cam needs several closely aligned bearings along the length of the cylinder head, which itself is rather squirmy under engine heat and load conditions. A radial cam usually has one lobe for each follower that it actuates, and each engine cylinder requires as many as five followers. The form of the cam lobes is convex, of a short radius, and the followers are also typically convex: their contact surfaces thus experience high Hertzian loads in operation. A radial cam is large and heavy, demanding in its bearing alignment, expensive in the forming of its many lobes, and suffers cam and follower contact surface degradation.
Modification of a cam's timing of its follower actuations, called “phasing,” is often desired with today's emphasis on engine emissions control and fuel efficiency. Phasing of a radial cam is done by interposing a mechanism that rotates with the cam, between the cam and its driving member. The mechanism is precisely made and closely regulated in operation, adding complexity to its bulk. A radial cam also needs careful attention paid to its surface lubrication, since lubricating oil tends to be flung from its surfaces. Thus, oil galleries are bored through the cam at additional expense.
An axial cam, often called a barrel cam, is short and can drive several followers from a single contact surface. As the axial cam rotates about its axis, the followers are driven axially by a contact surface of the cam. By incorporating another, oppositely facing, contact surface its followers can be driven in each axial direction by the cam. Axial cams are well known to be strong and long-enduring, and when employed to drive an engine's valve system require no bearings down the length of the cylinder head. Driven rods are deployed from the followers to operatively actuate the valves. With an axial cam, merely two contact surfaces need to be formed on it for the driving of its several followers. The cam's contact surfaces are concave where they cause the greatest acceleration to the followers, which are convex: their contact surfaces thus have minimal Hertzian loading. Phasing of an axial cam is done by modifying the mounted position of one or more of its followers about the cam's rotational axis. An axial cam embodiment is short and light weight, needs no special bearing alignment, is inexpensive to manufacture, and is strong and enduring. Minimal complexity and bulk are required for its phasing.
One form of an axial cam embodiment has the cam faces about the outside perimeter of its driving shaft. This is the most common form of axial cam, found in many applications beyond engine valve actuation. However, this form tends to be longer than desired because the driving shaft of the cam needs to itself be driven at one end, which adds length to the cam embodiment. Further adding to this length is the requirement that the cam needs to be stiff against deflection and thus extra axial thickness is given to the cam.
The present disclosure comprises an axial cam with oppositely facing cam surfaces fixed to a rotating exterior shell. The cam comprises a central cavity, within which at least a portion of each of its followers is mounted about the cam's rotational axis. The followers communicate their axial positions through an open end of the central cavity. Such a cam can be called an interior axial cam. The cam can be driven about its external perimeter with such as a belt drive or by gears, and its cam faces are given strength and stiffness by the exterior shell: thus, it is particularly compact axially. Lubricating oil forms a layer that is centrifugally flung against the inside surface of the rotating shell, contact between the oil layer and the followers splashing oil continuously against the cam contact faces.
The present disclosure is anticipated to be effective in operatively actuating those valves that ventilate positive displacement chambers in machines, whether in engines or in other devices such as compressors.